[FastISel][AArch64] Fix a few BuildMI callsites where the result register was...

[FastISel][AArch64] Fix a few BuildMI callsites where the result register was added as an operand register.

This fixes a few BuildMI callsites where the result register was added by
using addReg, which is per default a use and therefore an operand register.

Also use the zero register as result register when emitting a compare
instruction (SUBS with unused result register).

llvm-svn: 215997